Beat butter and brown and granulated sugars with a mixer on medium-high speed until pale and fluffy. Add eggs and yolk, 1 at a time, beating well after each addition and scraping down sides of bowl as needed. Beat in vanilla and molasses. Reduce speed to low. Gradually add flour mixture, and beat until just combined. Stir in white chocolate.

Spread batter into prepared pan. Bake until edges are golden, about 25 minutes. Let cool completely in pan on a wire rack. Cut into 2-inch squares or desired shape.